Sepsis strikes over one million in the US per year, with a mortality rate of 28-50%. Sepsis occurs when the host response to microbes extends beyond the site of infection and into the systemic circulation. This results in complex interactions involving microbes, blood components, and the endothelium that can progress to vascular collapse and organ failure. Neutrophils are key early responders to infection. Neutrophil myeloperoxidase (MPO) is a major mediator of oxygen-dependent microbicidal activity. MPO catalyzes the conversion of hydro- gen peroxide to hypochlorous acid, which reacts with both microbial and host molecular targets to produce a diverse array of oxidized lipids and metabolites. While neutrophil-derived oxidants promote microbe killing, they also have a critical role in host injury. For the proposed studies the complexity of host and polymicrobial mo- lecular targeting by neutrophil-derived oxidants will be investigated using both targeted and untargeted omics to identify new biomarkers, mediators and mechanisms of sepsis-associated multi-organ failure. Thus, we will test the hypothesis that neutrophil activation during sepsis leads to an oxidant-derived family of mole- cules that mediate sepsis-associated multi-organ failure and are candidate biomarkers of human sep- sis outcomes. This hypothesis will be tested with three specific aims. Specific Aim 1 will test the hypothesis that lipid and metabolic molecular signatures of neutrophil activation mediate endothelial dysfunction in human endothelial cells. There is a discovery arm to this aim to identify new molecular signatures of neutrophil activa- tion. Discovered molecular signatures as well as those revealed in our pilot studies will be tested as mediators of endothelial dysfunction and altered microbe killing. Specific Aim 2 will test the hypothesis that lipid and metabolic molecular signatures of neutrophil activation are biomarkers and mediators of multi-organ failure in rat sepsis. Pharmacological intervention will be used to examine the role of molecular signatures in outcomes of rat sepsis as determined by survival, endothelial dysfunction and multi-organ failure. Specific Aim 3 will test the hypothesis that novel molecular signatures of neutrophil activation associate with organ dysfunction and death in human sepsis. A multi-PI group has been assembled for the proposed studies with each PI having unique expertise. These PIs include the pioneer in the identification and biology of neutrophil-derived chlorin- ated lipids, Dr. Ford; an established clinician scientist with expertise in clinical sepsis studies, Dr. Nuala Meyer; and an expert in platelet, leukocyte, endothelial cell interactions and pathology, Dr. Jane McHowat. Overall, a multi-disciplinary approach will examine the role of oxidized lipids and metabolites produced as a result of neutrophil activation during sepsis as predictors of organ failure and mortality in human sepsis, and as media- tors of vascular endothelial dysfunction examined both in vivo in the rat and in vitro to test mechanisms of dysfunction. These innovative studies are designed to discover new paradigms for the role of neutrophils in eliciting endothelial dysfunction providing new targets for therapeutics to treat septic patients.